Under the current circumstances where large consumption of fossil fuels has continued and global warming tracerable to carbon dioxide and the like and air pollution in urban areas pose a serious concern, attention has been paid to hydrogen used as a next generation energy source in place of the fossil fuels. Since hydrogen discharges only water after the use thereof, this gas is considered as a clean energy source with a reduced environmental burden.
Although the most popular hydrogen producing technique is steam reforming of fossil fuels, there are known a diversity of other techniques such as side-produced hydrogen accompanied by the manufacture of iron or soda, thermal cracking reaction, photocatalytic reaction, microorganism reaction, electrolysis of water and the like. In particular, electric power necessary for the electrolysis of water can be supplied from a variety of sources and thus, the electrolysis is emphasized as an energy source that does not rely on specific areas. If electric power based on a reproducible energy such as from solar generation or wind generation is used, the discharge amount of carbon dioxide during power generation can be suppressed to a very small level.
For the electrolysis of water, there are known two techniques including a technique using a solid polymer membrane as an electrolyte and a technique using an alkaline aqueous solution. The solid polymer electrolysis using a solid polymer membrane as an electrolyte is advantageous in that a current density can be enhanced and involves a problem in that only carbon and noble metals are used as an electrode material because the membrane is strongly acidic. On the other hand, the alkaline electrolysis using an alkaline aqueous solution as an electrolyte has already shown satisfactory results over 70 years or more after practical use with good economy and thus, occupies most of the large-scale hydrogen producing apparatus.
The soft steel plated with Ni of high catalytic activity has been used mainly as an alkaline electrode, but with a problem in that ordinary Ni plating results in small surface irregularities, so that a specific surface area becomes so small as not to increase a current density. To cope with this problem, there have been developed techniques of providing an electrode surface with a high specific surface area, for example, a technique of electroforming a nanostructure on the surface using to a transfer plate technique of ordinarily roughening an electrode surface by use of a nanomold, a technique of shaping Ni fine particles into an electrode (e.g. Japanese Laid-open Patent Application No. 2002-317289), a technique of forming a metal serving as an electrode on a nanoporous structure (e.g. Japanese Laid-open Patent Application No. Hei 7-316862), and the like. Moreover, there have been developed a vacuum deposition process without resorting to a plating process and a surface treating method of nanostructures, such as nanofillers, nanoparticles and the like, by a dry process. In addition, the manufacture of electrodes by arc plating has also been proposed (e.g. Japanese Laid-open Patent Application No. 2005-15818).
With respect to electrodes for an alkaline water electrolyzer, there is a strong demand of developing a technique of enhancing a specific surface area so as to increase a current density.
The electrode for alkaline water electrolysis should have a certain area. This requires an increasing number of processes for the manufacture of a nanomold or nanoporous structure, and a vacuum apparatus is separately necessary for a system of creating a Ni film of high specific surface area in a dry process. In order to ensure low costs of a hydrogen producing apparatus of the water electrolysis type, it is essential to develop a process in which the Ni electrode surface with a large surface area can be readily formed so as to have a high specific surface area by a simple procedure.
The mere increase of specific surface area involves problems in that a solution resistance increases by adhesion of gas bubbles generated upon water electrolysis and thus, a current density cannot be increased and that the Ni structure of high specific surface area peels off by means of the gas pressure. Thus, it is necessary to provide a Ni structure that is not susceptible to an influence of the bubbles.
In order to overcome the above-stated problems, an object of the invention is to provide an electrode for electrolysis wherein a Ni electrode of a large surface area is simply enabled to have a high specific surface area and which is not susceptible to an influence of gas bubbles generated upon water electrolysis and also a method for manufacturing such an electrode.